In an inductively coupled plasma torch, referred to as an ICP torch, a gas is ionized in a plasma region located at one end of the torch and a sample is injected or introduced into the plasma region, the sample becoming atomized to enable elemental detection by a number of techniques, including spectrometric analysis. A prealigned demountable plasma torch is disclosed in applicant'U.S. Pat. No. 4,739,147, expressly incorporated herein by reference in its entirety.
To obtain optimal analytical performance with an ICP torch, particularly for interfacing spectrographic analysis with chromatography, it is desirable to introduce the sample into the plasma region in the form of a finely dispersed particle mist at a relatively constant flow. It is commonly accepted within the industry that the size of the sample particles reaching the plasma region should be less than about 15 microns. In order to prevent the introduction to the plasma region of larger sized particles, prior devices have utilized a nebulizer in conjunction with some type of large volume particle size discrimination chamber.
For systems of this type, a liquid sample to be analyzed and an aerosol gas are fed into a nebulizer mounted within the large volume chamber, which, for example, by means of double pass baffling systems, produces an aerosol mixture of sample and gas. The chamber is connected by a suitable joint for fluid communication with a first end of an internal sample pass tube of an ICP torch. The tube extends through the ICP torch and has a second end terminating adjacent the plasma region, wherein subsequent atomization of the sample occurs.
To reach the plasma region, and eventual atomization, the sample must pass from the nebulizer, through the large volume container and through the internal tube. At typical operating pressures and gas flow rates, only the smaller sized particles remain suspended long enough to traverse this route. The relatively large droplets, i.e., those greater than 15 microns, that exit the nebulizer contact and condense upon the walls of the spray chamber, for collection through an outlet as waste. A large volume discrimination chamber of this type has a volume of about 125 milliliters (cubic centimeters). The liquid sample can be pumped to the nebulizer by a peristaltic pump at a rate of between 0.1 and 1 milliliter per minute, while the aerosol gas is input to the nebulizer at a pressure ranging from about 15-30 psi, resulting in a flow rate of gas into the nebulizer of about one liter per minute.
Systems of this type generally provide a relatively slow, steady flow of sample into the plasma region, which results in increased residence time of the sample while in the plasma region. Increased residence time produces more thorough atomization of the sample particles, and thus is desirable for clarity in analysis. However, due to the relatively slow sample flow rate, which for these systems is directly dependent upon the nebulizer pressure setting, only about 1 to 3% of the liquid sample introduced into the nebulizer actually reaches the plasma region. In other words, there is a trade off between clarity and efficiency, and the nebulizer pressure setting must be chosen so as to affect a compromise between these two concerns. An increase in pressure will result in an increase in the percentage of sample that is introduced to the plasma region, but a decrease in the effective atomization of the particles that are introduced.
This concern becomes particularly acute when organic samples are to be tested. Organic sample particles tend to be heavier then corresponding inorganic sample particles, and thus require a higher operating pressure in order to overcome the effects of gravity and remain suspended long enough to reach the plasma region. However, increased pressure may produce excessive formation of vapor together with the sample particles, which can adversely affect the clarity of spectral lines during testing.
In addition to low efficiency, the relatively large "dead volume" of the spray chamber makes it more difficult to maintain clarity during high resolution chromatography, in which the intensity of the discharge is measured in the time domain. Typically, in liquid chromatography, a pure liquid sample of several compounds undergoes a size exclusion step prior to being introduced to the nebulizer of the ICP torch. This size exclusion step separates the compounds according to molecular size. Thus, the different compounds successively pass through the torch for ionization in the plasma region, according to molecular size, whereupon they are analyzed during the time period in which they are in the plasma region. In high resolution chromatography, it is important to distinguish the different compounds, or to detect the transition between successive compounds. However, because the large "dead volume" occupied by the input sample tends to slow sample flow, it becomes difficult to visually discern the transition between a first atomized compound and the compound atomized immediately thereafter.
The relatively large volume chamber also results in lost time during testing. When changeover to another sample is required, the relatively large volume spray chamber must be thoroughly cleaned in order to assure the accuracy of any subsequent testing. Then, before testing starts up again, the entire discrimination chamber volume must be filled before any of the sample reaches the plasma region. Ultimately, for a large number of tests the cumulative time required to clean and to refill the large volume spray chamber represents a substantial loss in testing time.
Others have attempted to overcome the disadvantages associated with large volume chamber systems of this type. For example, Fassel et al. U.S. Pat. No. 4,575,609 is directed to a direct injection nebulizer that is designed to overcome dead volume problems associated with prior systems. This is partially accomplished by maintaining an aerosol flow through the nebulizer orifice into the plasma region at a velocity not less than 100 meters per second. According to this patent, a micro nebulizer that extends through the torch has inner and outer concentric tubes that terminate just short of the plasma region. Liquid solvent and a source of sample liquid are pumped through the inner tube, while nebulizing gas is pumped through the outer tube. With this structure, nebulization occurs adjacent to, or within, the plasma region and just prior to atomization. Nearly 100% of the sample enters the plasma region. Compared to the prior large volume systems, the smaller volume, less than about 5 microliters, and preferably about 2.5 microliters, and the increased velocity at the orifice, about b 100 m/s, reduce problems associated with dead volume.
There are practical disadvantages associated with direct injection nebulizers of this type. In order to maintain an aerosol flow rate of about 100 meters per second, the pump must be operated at a relatively high pressure ranging from about 100 to 1000 psi. A typical peristaltic pump used for the prior large volume systems is not capable of operating under such high nebulizer back pressures, and thus, new equipment must be purchased in order to operate the direct injection nebulizer. Additionally, operation costs are increased due to the increased energy expended in operating a pump at these relatively high pressures. The micro nebulizer is also extremely susceptible to breakage, due to the fact that the outer tube is tapered to within a distance of less than 0.05 mm from the inner tube.
The direct injection nebulizer also suffers in analytical performance characteristics. Due to the high flow rate, the residence time of sample particles in the plasma region is reduced. The relatively high flow rate, the small volume tube and the micro nebulizer orifice also combine to produce excessive sample turbulence, sometimes referred to as flicker, within the plasma region. This flicker affects the magnitude of the emission signal that is sensed by the spectrometer. Additionally, because the micro nebulizer orifice is located directly adjacent to the plasma region, the direct injection nebulizer does not discriminate sample particles by size. There is no assurance whatsoever that sample particles being introduced into the plasma region have a size of less than 15 microns. In fact, most particles are not within the desired size range.
In other words, while a direct injection nebulizer of this type may reduce some of the efficiency problems associated with large volume discrimination chamber systems, it also introduces a number of other problems, both practical and analytical.
It is an object of this invention to provide an ICP torch that promotes the efficient introduction of small sized sample particles into a plasma region at a constant flow rate, while at the same time overcoming disadvantages associated with prior large volume discrimination chamber systems and with direct injection nebulizer systems.